This application proposes experiments intended to develop improved behavioral methods for categorizing abused drugs, and/or for determining the abuse liability of drugs. All are based on the drug discrimination technique. Experiment 1 will test for the occurrence of stimulus masking in the drug discrimination preparation, to determine whether this is a frequent or rare phenomenon. Experiment 2 will test for the occurrence of cue overlap between the sensory effects of drugs which produce pharmacologically dissimilar actions. Experiment 3 will determine the degree to which the specificity of the DD paradigm varies as a function of the training dosage employed. Experiment 4 involves training with a mixture of two different drug cues in order to determine whether perception of drug stimuli is analytic or synthetic in nature, and to investigate individual differences between animals i cue usage when more than one cue is available. Experiment 5 investigates the degree to which the drug stimuli present at the time of onset of drug actions differ from those which occur later. Experiments 6-8 will investigate the utility of a novel multiple-drug training procedure which is designed to allow concurrent discrimination of several training drugs. A total of 8 separate experiments are proposed using rats and pigeons as subjects.